​

Python syntax was made for readability, and easy editing. For example, the python language uses a : and indented code, while javascript and others generally use {} and indented code.

Lets create a python 3 repl, and call it Hello World. Now you have a blank file called main.py. Now let us write our first line of code:

helloworld.py

print('Hello world!')

Brian Kernighan actually wrote the first “Hello, World!” program as part of the documentation for the BCPL programming language developed by Martin Richards.

Now, press the run button, which obviously runs the code. If you are not using replit, this will not work. You should research how to run a file with your text editor.

If you look to your left at the console where hello world was just printed, you can see a >, >>>, or $ depending on what you are using. After the prompt, try typing a line of code.

Python 3.6.1 (default, Jun 21 2017, 18:48:35)
[GCC 4.9.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
> print('Testing command line')
Testing command line
> print('Are you sure this works?')
Are you sure this works?
>

The command line allows you to execute single lines of code at a time. It is often used when trying out a new function or method in the language.

Another cool thing that you can generally do with all languages, are comments. In python, a comment starts with a #. The computer ignores all text starting after the #.

shortcom.py

# Write some comments!

If you have a huge comment, do not comment all the 350 lines, just put ''' before it, and ''' at the end. Technically, this is not a comment but a string, but the computer still ignores it, so we will use it.

longcom.py

'''
Dear PYer,
I am confused about how you said you could use triple quotes to make
SUPER
LONG
COMMENTS
!
​
I am wondering if this is true,
and if so,
I am wondering if this is correct.
​
Could you help me with this?
​
Thanks,
Random guy who used your tutorial.
'''
print('Testing')

Unlike many other languages, there is no var, let, or const to declare a variable in python. You simply go name = 'value'.

vars1.py

x = 5
y = 7
z = x*y # 35
print(z) # => 35

Remember, there is a difference between integers and strings. Remember: String = "". To convert between these two, you can put an int in a str() function, and a string in a int() function. There is also a less used one, called a float. Mainly, these are integers with decimals. Change them using the float() command.

vars2.py

x = 5
x = str(x)
b = '5'
b = int(b)
print('x = ', x, '; b = ', str(b), ';') # => x = 5; b = 5;

Instead of using the , in the print function, you can put a + to combine the variables and string.

There are many operators in python:

• +

• -

• /

• * These operators are the same in most languages, and allow for addition, subtraction, division, and multiplicaiton. Now, we can look at a few more complicated ones:

• %

• //

• **

• +=

• -=

• /=

• *= Research these if you want to find out more…

simpleops.py

x = 4
a = x + 1
a = x - 1
a = x * 2
a = x / 2

You should already know everything shown above, as it is similar to other languages. If you continue down, you will see more complicated ones.

complexop.py

a += 1
a -= 1
a *= 2
a /= 2

The ones above are to edit the current value of the variable. Sorry to JS users, as there is no i++; or anything.

Fun Fact: The python language was named after Monty Python.

If you really want to know about the others, view Py Operators​

Like the title? Anyways, a ' and a " both indicate a string, but do not combine them!

quotes.py

x = 'hello' # Good
x = "hello" # Good
x = "hello' # ERRORRR!!!

slicing.py

String Slicing

You can look at only certain parts of the string by slicing it, using [num:num]. The first number stands for how far in you go from the front, and the second stands for how far in you go from the back.

x = 'Hello everybody!'
x[1] # 'e'
x[-1] # '!'
x[5] # ' '
x[1:] # 'ello everybody!'
x[:-1] # 'Hello everybod'
x[2:-3] # 'llo everyb'

Methods and Functions

Here is a list of functions/methods we will go over:

• .strip()

• len()

• .lower()

• .upper()

• .replace()

• .split()

I will make you try these out yourself. See if you can figure out how they work.

strings.py

x = " Testing, testing, testing, testing       "
print(x.strip())
print(len(x))
print(x.lower())
print(x.upper())
print(x.replace('test', 'runn'))
print(x.split(','))

Good luck, see you when you come back!

Input is a function that gathers input entered from the user in the command line. It takes one optional parameter, which is the users prompt.

inp.py

print('Type something: ')
x = input()
print('Here is what you said: ', x)

If you wanted to make it smaller, and look neater to the user, you could do…

inp2.py

print('Here is what you said: ', input('Type something: '))

Running: inp.py

Type something:
Hello World
Here is what you said: Hello World

inp2.py

Type something: Hello World
Here is what you said: Hello World

Python has created a lot of functions that are located in other .py files. You need to import these modules to gain access to the,, You may wonder why python did this. The purpose of separate modules is to make python faster. Instead of storing millions and millions of functions, , it only needs a few basic ones. To import a module, you must write input <modulename>. Do not add the .py extension to the file name. In this example , we will be using a python created module named random.

module.py

import random

Now, I have access to all functions in the random.py file. To access a specific function in the module, you would do <module>.<function>. For example:

module2.py

import random
print(random.randint(3,5)) # Prints a random number between 3 and 5

Pro Tip: Do from random import randint to not have to do random.randint(), just randint() To import all functions from a module, you could do from random import *

Loops allow you to repeat code over and over again. This is useful if you want to print Hi with a delay of one second 100 times.

for Loop

The for loop goes through a list of variables, making a seperate variable equal one of the list every time. Let’s say we wanted to create the example above.

loop.py

from time import sleep
for i in range(100):
     print('Hello')
     sleep(.3)

This will print Hello with a .3 second delay 100 times. This is just one way to use it, but it is usually used like this:

loop2.py

import time
for number in range(100):
     print(number)
     time.sleep(.1)

while Loop

The while loop runs the code while something stays true. You would put while <expression>. Every time the loop runs, it evaluates if the expression is True. It it is, it runs the code, if not it continues outside of the loop. For example:

while.py

while True: # Runs forever
     print('Hello World!')

Or you could do:

while2.py

import random
position = '<placeholder>'
while position != 1: # will run at least once
    position = random.randint(1, 10)
    print(position)

The if statement allows you to check if something is True. If so, it runs the code, if not, it continues on. It is kind of like a while loop, but it executes only once. An if statement is written:

if.py

import random
num = random.randint(1, 10)
if num == 3:
     print('num is 3. Hooray!!!')
if num > 5:
     print('Num is greater than 5')
if num == 12:
     print('Num is 12, which means that there is a problem with the python language, see if you can figure it out. Extra credit if you can figure it out!')

Now, you may think that it would be better if you could make it print only one message. Not as many that are True. You can do that with an elif statement:

elif.py

import random
num = random.randint(1, 10)
if num == 3:
    print('Num is three, this is the only msg you will see.')
elif num > 2:
    print('Num is not three, but is greater than 1')

Now, you may wonder how to run code if none work. Well, there is a simple statement called else:

else.py

import random
num = random.randint(1, 10)
if num == 3:
    print('Num is three, this is the only msg you will see.')
elif num > 2:
    print('Num is not three, but is greater than 1')
else:
    print('No category')

So far, you have only seen how to use functions other people have made. Let use the example that you want to print the a random number between 1 and 9, and print different text every time. It is quite tiring to type:

Characters: 389

nofunc.py

import random
print(random.randint(1, 9))
print('Wow that was interesting.')
print(random.randint(1, 9))
print('Look at the number above ^')
print(random.randint(1, 9))
print('All of these have been interesting numbers.')
print(random.randint(1, 9))
print("these random.randint's are getting annoying to type")
print(random.randint(1, 9))
print('Hi')
print(random.randint(1, 9))
print('j')

Now with functions, you can seriously lower the amount of characters:

Characters: 254

functions.py

import random
def r(t):
     print(random.randint(1, 9))
     print(t)
r('Wow that was interesting.')
r('Look at the number above ^')
r('All of these have been interesting numbers.')
r("these random.randint's are getting annoying to type")
r('Hi')
r('j')

Chapter 01 - Getting Ready with Python

Installing Python 3, And Launching Python Shell

This video should help you get up and running with Python 3

• ​Installing Python 3 and Launch Python Shell​

Installing Python is really a cakewalk. Search for “Python download” on www.google.com. Download the installable and install it.

A quick word of caution on Windows

• Make sure that you have the check-box “Add Python 3.6 to PATH”, checked.

Once you have installed Python, you can launch the Python Shell.

• Windows - Launch cmd prompt by typing in ‘cmd’ command.

• Mac or Linux - Launch up terminal.

Command to launch Python 3 is different in Mac.

• In Mac, type in python3

• In other operating systems, including windows, type python

You can type code in python shell and code as well!

You can use print(5*4), and it shows 20.

You can execute the code, and the shell would immediately give you output.

Using the the Python Shell is an awesome way to learn Python.

Chapter 02 - Introduction To Python Programming

Most programmers find programming a lot of fun, and besides, it also gets their work done.

Programming mainly involves problem solving, where one makes use of a computer to solve a real world problem.

During our journey here, we will approach programming in a very different way. We will not only introduce you to the Python language, but also help you pick up essential problem solving skills.

As a programmer, you need to be able to look at a problem, and identify the important programming concepts relevant to solving it. Finally, you need to be able to use the language features and syntax, to express your solution on the computer. While all this looks complex, we want to make it easy for you. Together, we will tackle a variety of programming challenges, using these same steps. We will start with simple challenges (such as a Multiplication Table), and gradually increase the difficulty level over the duration of this book.

Learning to program is a lot like learning to ride a bicycle. The first few steps are the most challenging ones.

Once you get over these initial steps, your experience will become more and more enjoyable.

Are you ready for your first programming challenge? Let’s get going now! We wish you all the best.

Summary

In this step, we:

• Were introduced to the concept of problem solving

• Understood how good programmers approach problem solving

Step 01: Our First Programming Challenge

Our first programming challenge aims to do, what every kid does in math class: read out a multiplication table. We now want to give this task to the computer. Here is the statement of our problem:

The Print Multiplication Table Challenge (PMT-Challenge)

1. Compute the multiplication table for 5, with entries from 1 to 10.

2. Display this table.

The display needs to be:

5 * 1 = 5

5 * 2 = 10

5 * 3 = 15

5 * 4 = 20

5 * 5 = 25

5 * 6 = 30

5 * 7 = 35

5 * 8 = 40

5 * 9 = 45

5 * 10 = 50

This is the challenge. For convenience, let’s give it a label, say PMT-Challenge. What would be the important concepts we need to learn, to solve this challenge? The following list of concepts would be a good starting point:

• Statements

• Expressions

• Variables

• Literals

• Conditionals

• Loops

• Methods

In the rest of this chapter, we will introduce these concepts to you, one-by-one. We will also show you how learning each concept, takes us closer to a solution to PMT-Challenge.

Summary

In this step, we:

• Stated our first programming challenge

• Identified what programming concepts we need to learn, to solve this challenge

Step 02: Breaking Down PMT-Challenge

Typically when we do programming, we have problems. Solving the problem typically need a step-by -step approach. Common sense tells us that to solve a complex problem, we break it into smaller parts, and solve each part one by one. Here is how any good programmer worth her salt, would solve a problem:

• Simplify the problem, by breaking it into sub-problems

• Solve the sub-problems in stages (in some order), using the language

• Combine these solutions to get a final solution

The PMT-Challenge is no different! Now how do we break it down, and where do we really start? Once again, your common sense will reveal a solution. As a first step, we could get the computer to calculate say, 5 * 3. The second thing we can do, is to try and print the calculated value, in a manner similar to 5 * 3 = 15. Then, we could repeat what we just did, to print out all the entries of the 5 multiplication table. Let’s put it down a little more formally:

Here is how our draft steps look like

• Calculate 5 * 3 and print result as 15

• Print 5 * 3 = 15 (15 is result of previous calculation)

• Do this ten times, once for each table entry (going from 1 to 10)

Let’s start with that kind of a game plan, and see where it takes us.

Summary

In this step, we:

• Learned that breaking down a problem into sub-problems is a great help

• Found a way to break down the PMT-Challenge problem

Step 03: Introducing Operators And Expressions

Let’s focus on solving the first sub-problem of PMT-Challenge, the numeric computation. We want the computer to calculate 5 * 5 for example, and print 25 for us. How do we get it to do that? That’s what we would be looking at in this step.

Snippet-01: Introducing Operators

Launch up Python shell. We want to calculate 5 * 5. How do we do that?

Using our knowledge of school math, let’s try 5 X 5.

    >>> 5 X 5
    File "< stdin >", line 1
    5 X 5
      ^
    SyntaxError: invalid syntax

The Python Shell hits back at us, saying “invalid syntax”. This is how Python complains, when it doesn’t fully understand the code you type in. Here, it says our code has a “SyntaxError”.

The reason why it complains, is because ‘X’ is not a valid operator in Python.

The way you can do multiplication is by using the ‘*’ operator .

“5 into 5” is achieved by the code 5 * 5, and you can see the result 25 being printed. Similarly, 5 * 6 gives us 30.

    >>> 5 * 6
    30

There are a wide range of other operators in Python:

• 5 + 6 gives a result of 11.

• 5 - 6 leads to -1.

  >>> 5 + 6
  11
  >>> 5 - 6
  -1

10 / 2, gives an output of 5.0 . There is one interesting operator, **. Let’s try 10 ** 3. We ran this code, and the result we get is 1000. Yes you guessed right, the operator performs “to the power of”. “10 to the power of 3” is 10 * 10 * 10, or 1000.

    >>> 10 / 2
    5.0
    >>> 10 ** 3
    1000

Another interesting operator is %, called “modulo”, which computes the remainder on integer division. If we do 10 % 3, what is the remainder when 10 is divided by 3? 3 * 3 is 9, and 10 - 9 is 1, which is what % returns in this case.

Let’s look at some terminology:

• Whatever pieces of code we gave Python shell to run, are called expressions. So, 5 * 5, 5 * 6 and 5 - 6 are all expressions. An expression is composed of operators and operands.

• In the expression 5 * 6, the two values 5 and 6 are called operands, and the * operator operates on them.

• The values 5 and 6 are literals, because those are constants which cannot be changed.

The cool thing about Python, is that you can even have expressions with multiple operators. Therefore, you can form an expression with 5 + 5 + 5, which evaluates to 15. This is an expression which has three operands, and two + operators. You can even have expressions with different types of operators, such as in 5 + 5 * 5.

    >>> 5 + 5 + 5
    15
    >>> 5 + 5 * 5
    30

Try and play around with the expressions, and understand the output which results.

Summary

In this step, we:

• Learned how to give code input to the Python Shell

• Understood that Python has a predefined set of operators

• Used a few types of basic operators and their operands, to form expressions

Step 04: Programming Exercise IN-PE-01

At this stage, your smile tells us that you enjoy evaluating Python expressions. What if we tickle your mind a bit, to make sure it hasn’t fallen asleep? Here is your first programming exercise.

Exercises

1. Write an expression to calculate the number of minutes in a day.

2. Write an expression to calculate the number of seconds in a day.

Note

You need to solve these problems by yourself. If you are able to work them out, that’s fantastic! But if not, that’s part of the learning process.

Solutions

Solution 1

    >>> 24 * 60
​
    1440

We wanted to calculate the number of minutes in a day. How do we do that? Think about this…

• How many number of hours are there in a day? 24.

• And how many minutes does each hour have? It’s 60.

• So if you want to find out the number of minutes in a day, it’s 24 * 60, which is 1440.

Solution 2

    >>> 24 * 60 * 60
​
    86400

How many seconds are there in a day?

• Let’s start with the number of hours, 24.

• The number of minutes in an hour is 60, and

• The number of seconds in a minute is 60 as well.

• So it’s 24 * 60 * 60, or 86400.

Summary

In this step, we:

• Solved a Programming Exercise involving common scenarios, using Python code involving:

  • Expressions

  • Operators

  • Literals

Step 05: Puzzles On Expressions

Let’s look at a few puzzles related to expressions, in this step. Before that, let’s revise some of the terminology we had learned earlier.

5 + 6 + 10 is an example of an expression. In this expression, 5, 6 and 10 are operands. The + here is the operator. You can have multiple operators in an expression. We also did mention that the operands, namely 10, 6 and 5, are literals. Their values will not change.

Here are a few puzzles coming up, to explore aspects of expressions.

Snippet-01: Puzzles On Expressions

Think about what would happen when you do something of this kind: 5 $ 2. You’re right, it would throw a SyntaxError. When Python does not understand the code you type in, it reports an error. Here, the expression we’re typing is 5 $ 2, which does not make sense to Python, hence the SyntaxError.

    >>> 5 $ 2
    File "< stdin >", line 1
    5 $ 2
    ^
    SyntaxError: invalid syntax
    >>> 5$2
    File "< stdin >", line 1
    5 $ 2
    ^
    SyntaxError: invalid syntax

Let’s say we type in 5+6+10, without any spaces between the operands, and the operators. What do you think will happen? Surprisingly, the Python Shell does calculate the value!

    >>> 5+6+10
    21

In an expression, using spaces makes it easier for you to read it, but it’s not mandatory. 5 + 6 + 10 is easier to read than 5+6+10, but does not make any difference to the Python compiler.

The next puzzle tries to evaluate 5 / 2, which is “5 divided by 2”. What would be the output? 2.5.

    >>> 5/2
    2.5

If you’re coming from other programming languages like Java or C, this might be a surprising result. If you try this in Java for instance, you would get 2 as the output. Note that even though both operands are integers, the result of the / operation is a floating point value, 2.5 . Python does what is expected by a programmer!

The puzzle after that tries to play with 5 + 5 * 6. What would be the result of this expression? Will it be 5 + 5 or 10, then 10 * 6, which is 60? Or, will it be 5 plus 5 * 6, which is 5 + 30, that’s 35?

    >>> 5 + 5 * 6
    35

The correct result is 35.

Python decides this is based on the precedence of operators.

Operators in Python are divided into two sets as follows:

• **, *, / and % have higher precedence, or priority.

• + and - have a lower precedence.

Sub-expressions involving operators from {*, /, %, **} are evaluated before those involving operators from {+, -}

Let’s try another small puzzle on precedence, with 5 - 2 * 2. What would be the result of this? Will it be 6, or 1? It’s 1, because * has a higher precedence than -. Thus 2 * 2 is 4, and 5 - 4 gives us 1.

    >>> 5 - 2 * 2
    1

Let’s say we want to execute 5 - 2, to give an output of 2. How do we change the operator precedence?

You cannot really change the precedence, but you can add parentheses to group sub-expressions differently.

    >>> (5 - 2) * 2
    6
    >>> 5 - ( 2 * 2 )
    1

Parentheses have the highest precedence in Python, and can be used to override operator precedence. (5 - 2) gets calculated first, and the final result of the expression is 6.

A positive thing about using parentheses is, that it makes expressions more readable. So even in situations such as 5 - 2 * 2, where we know the result according to precedence, adding parentheses is good.

Summary

In this step, we went about solving a few puzzles about expressions, touching concepts such as:

• SyntaxError for incorrect operators

• White-space in expressions

• Floating Point division by default

• Operator Precedence

• Using parentheses

Step 06: Printing Text

In the previous step, we learned how to use expressions to compute values. In this step, let’s see how we can actually print multiplication table entries, that are readable by the user.

Snippet-01: Printing Text

How do we go about printing a complete multiplication table entry? We want to print text such as 5 * 6 = 30 . But trying to do so, as we know it, gives us a SyntaxError. Clearly, there is a different way to print text, as compared to an expression.

    >>> 5 * 6 = 30
      File "<stdin>", line 1
    SyntaxError: can't assign to operator

Let’s first try to print a simple piece of text, Hello. Typing in this piece of code directly on Python Shell also gives us an error.

    >>> Hello
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Hello' is not defined

Only expressions work that way, and Hello is not really an expression.

"Hello" is typically called a string, and represents the text of letters 'H', 'e', 'l', 'l', 'o'. "Hello" is hence different from the number 5.

There are a number of in-built functions in Python to help print strings. One of these is the print() function. Can you just say print Hello?

    >>> print Hello
      File "<stdin>", line 1
        print Hello
                  ^
    SyntaxError: Missing parentheses in call to 'print'. Did you mean print(Hello)?

The Python compiler gives you an error, that says “missing parentheses”.

Will print(Hello) work?

    >>> print (Hello)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Hello' is not defined

Nope! Again, this one failed because you need to indicate that "Hello" is a string.

How do I indicate that "Hello" is a string? By putting it within double quotes.

Let’s try print ("Hello")

    >>> print ("Hello")
    Hello
    >>> print("Hello")
    Hello

print("Hello") finally results in "Hello" being printed out. To be able to print "Hello", the things we need to do are:

• Typing the method name print ,

• open parentheses ( ,

• Followed by a double quote " ,

• The text Hello,

• and another double quote " ,

• finished off with a closed parentheses ).

What we have written here is called a statement, a simple piece of code to execute. As part of this statement, we are calling a function, named print().

What exactly are we trying to print?

The text "Hello", which is called a parameter or argument, to print().

Now let’s get back to what we wanted to do, which is to print 5 * 6 = 30. The most basic version would be something of this kind, print("5 * 6 = 30"). Here, we are passing the entire value in the form of a string.

    >>> print("5 * 6 = 30")
    5 * 6 = 30

This prints the text on the console, as-is. The thing you need to understand here is, we aren’t really calculating 30 using the formula 5 * 6, but directly putting text 30 in here. That’s called hard-coding.

In a later step, we will look at how to actually calculate the value and pass it in.

Summary

In this step, we:

• Understood that displaying text on the console is not the same as printing an expression value

• Learned about the print() function, that is used to print text in Python.

• Found a way to print the text "5 * 6 = 30" on the console, by hard-coding values in a string

Step 07: Puzzles On Utility Methods, And Strings

In the previous step, we learned how to print 5 * 6 = 30. It was not a perfect solution, because we hard-coded everything. we used an in-built function named print(), passed a string to it, and invoked the method.

In this step, let’s look at a number of puzzles related to in-built methods, their parameters, and strings in general.

For example, let’s do print("5 * 6"), as in the previous step. What does this code result in?

    >>> print("5*6")
    5*6
    >>> print('5*6')
    5*6

It just prints the string "5 * 6".

Let’s say we try the code print(5 * 6),

    >>> print(5*6)
    30

Without the double quotes, 5 * 6 is an expression. What will be the output? 30.

If you call print() with an expression argument, it prints the value of the expression. However, when we pass something within double quotes, it becomes a piece of text, printed as-is.

An interesting thing to note is, that in Python you can use either double-quotes (" and "), or single-quotes (' and ') with text values.

Let’s look at a few other in-built methods within Python.

Consider abs() (which stands for absolute value), a method that accepts a numeric value. You can use abs(10.5), passing 10.5 as a value to it, and it prints the absolute value of 10.

    >>> abs 10.5
      File "<stdin>", line 1
        abs 10.5
               ^
    SyntaxError: invalid syntax
    >>> abs(10.5)
    10.5

If you pass in a string value, will it work? It complains, “abs() function will not work with a string, it only works with numeric values”.

    >>> abs("10.5")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: bad operand type for abs(): 'str'

Let’s say you want to use a function that computes “to the power of”, for instance “2 to the power of 5”. In Python, there’s an in-built function named pow(), which does just what we need. To pow(), you can pass two parameters and calculate the result. How do you do that?

Will this work: pow 2 5? No, not at all. This code does not work as well: pow(2 5). pow(2, 5) is the correct syntax.

    >>> pow 2 5
      File "<stdin>", line 1
        pow 2 5
            ^
    SyntaxError: invalid syntax
    >>> pow(2 5)
      File "<stdin>", line 1
        pow(2 5)
              ^
    SyntaxError: invalid syntax
    >>> pow(2, 5)
    32

You’ll see that 32 is printed.

Let’s see another example, “10 to the power of 3”. pow(10,3) is the alternative to saying 10 ** 3. This gives us 1000, similar to how pow() would.

    >>> pow(10, 3)
    1000
    >>> 10 ** 3
    1000

max() returns maximum in a set of numbers.min() function returns the minimum value.

    >>> max(34, 45, 67)
    67
    >>> min(34, 45, 67)
    34

These are some of the in-built functions in Python, and we saw how to call the in-built functions by passing in a varied number of parameters.

Python is case sensitive. So let’s say I want of calculate pow(2,5). So this would give me 32. Now, what if I say capital 'P' instead of small 'p' here? Pow(2,5) would lead to an error.

    >>> pow(2,5)
    32
    >>> Pow(2,5)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Pow' is not defined

The only things not case-sensitive in Python, are string values. Earlier we saw that the code print("Hello") displays the text "Hello". Inside a string, the text can be in any case. Hence, print("hello") displays "hello" ,with a small 'h'.

    >>> print("Hello")
    Hello
    >>> print("hello")
    hello
    >>> print("hellO")
    hellO
    >>> print ( "hellO" )
    hellO

However inside your code, you need to be very particular about the case of function names, class names, variable names, and the like.

In your code, whitespace does not really matter. You can add space here and here, and you would still get the same output. However, in case of strings, whitespace does matter.

If we say print("hellO World"), it would print "hellO World", with a space in between. And if you do print("hellO World") with three spaces, it would print the same. In expressions, white-space does not affect the output.

    >>> print ( "hellO World" )
    hellO World
    >>> print ( "hellO     World" )
    hellO     World

The last thing we want to look at, is an escape sequence. Let’s say you want to print a double quote, ", in the code. If we were to do this: print("Hello""), what would happen? The compiler says error!

    >>> print("Hello"")
      File "<stdin>", line 1
        print("Hello"")
                      ^
    SyntaxError: EOL while scanning string literal

If you want to print a " inside a string, use an escape sequence. In Python, the symbol '\' is used as an escape character. On using '\' adjacent to the ", it prints Hello" (notice the trailing "). We have used the '\' to escape the ", by forming an escape sequence \".

>>> print("Hello\"")
Hello"
>>>

The other reason why you would want to use a '\' is to print a <NEWLINE>. If you want to print "Hello World", but with "Hello" on one line and "World" on the next, '\n' is the escape sequence to use.

    >>> print("Hello\nWorld")
    Hello
    World

The other important escape sequence is '\t', which prints a <TAB> in the output. When you do print("Hello\tWorld"), you can see the tab-space between "Hello" and "World".

    >>> print("Hello\tWorld")
    Hello   World

Another useful escape sequence is \\ . If you want to print a \ , then use the sequence \\ . You would see that it prints Hello\World . Think about what would happen if we put six \ . Yes you’re right! It would print this string: "\\\" .

    >>> print("Hello\\World")
    Hello\World
    >>> print("Hello\\\\\\World")
    Hello\\\World

One of the things with Python is, it does not matter whether you use double quotes or single quotes to enclose strings. There are some interesting, and useful ways of using a combination of both, within the same string. Have a look at this call: print("Hello'World"), and notice the output we get. In a similar way, the following code will be accepted and run by the Python system: print('Hello"World').

    >>> print('Hello"')
    Hello"
    >>> print("Hello'World")
    Hello'World
    >>> print("Hello\"World")
    Hello"World
    >>> print("Hello\"World")
    Hello"World

The above two examples can be used as a tip by newbie programmers when they form string literals, and want to use them in their code:

• If the string literal contains one or more single quotes, then you can use double quotes to enclose it.

• However if the string contains one or more double quotes, then prefer to use single quotes to enclose it.

Summary

In this step, we:

• Explored a number of puzzles related to code involving:

  • Built-in functions for numeric calculations

  • The print() function to display expressions and strings

• Covered the following aspects of the above utilities:

  • Case-sensitive aspects of names and strings

  • The role played by whitespace

  • The escape character, and common escape sequences

Step 08: Formatted Output With print()

In the previous step, we learned how to print a hard-coded string, such as "5 * 6 = 30".

In this step, let’s try to replace the hard-coded 30 with a computed value.

Let’s start with a simple scenario. Let’s say we want to place that calculated value within a string, and display it. How do we do that?

Snippet-01: print() Formatted Output

format() method can be used to print formatted text.

Let’s see an example:

    >>> print("VALUE".format(5*2))
    VALUE

We were expecting 10 to be printed, but it’s actually printing VALUE.

How do we get 10 to be printed then?

    >>> print("VALUE {0}".format(5*2))
    VALUE 10

By having an open brace {, closed brace }, and and by putting the index of the value between them. Here, the value is the first parameter, and it’s index will be 0.

"VALUE {0}" is what we need.

Let’s take another example. Suppose to the format() function, we pass three values: 10, 20 and 30.

Typically when we count positions or indexes, we start from 0.

To print the first value, you need to pass in an index of 0. To print the second value, pass an index of 1.

    >>> print("VALUE {0}".format(10,20,30))
    VALUE 10
    >>> print("VALUE {1}".format(10,20,30))
    VALUE 20
    >>> print("VALUE {2}".format(10,20,30))
    VALUE 30

Now going back to our problem, we wanted to display "5 * 6 = 30", but without hard-coding. Instead of 30, we want the calculated value of 5 * 6.

    >>> print("5 * 6 = 30".format(5,6,5*6))
    5 * 6 = 30

Let replace "5 * 6 = 30" with "5 * 6 = {2}". 2 is the index of parameter value 5*6.

    >>> print("5 * 6 = {2}".format(5,6,5*6))
    5 * 6 = 30

Cool! Progress made.

Let’s replace 5 * 6 with the right indices - {0} * {1}.

    >>> print("{0} * {1} = {2}".format(5,6,5*6))
    5 * 6 = 30

The great thing about this, is now we can replace the values we passed to print() in the first place, without changing the indexes! So, we can display results for 5 * 7 = 35 and 5 * 8 = 40. We are now able to print 5 * 6 = 30, 5 * 7 = 35, 5 * 8 = 40, and can do similar things for other table entries as well.

    >>> print("{0} * {1} = {2}".format(5,7,5*7))
    5 * 7 = 35
    >>> print("{0} * {1} = {2}".format(5,8,5*8))
    5 * 8 = 40
    >>> print("{0} * {1} = {2}".format(5,8,5*8))
    5 * 8 = 40

Summary

In this step, we:

• Discovered that Python provides a way to do formatted printing of string values

• Looked at the format() function, and saw how to call it within print()

• Observed how we could work only with the indexes of parameters to format(), and change the parameters we pass without changing the code

Step 09: Puzzles On format() and print()

In this step, let’s look at a few puzzles related to the format, and the print methods.

Snippet-01: format() And print() Puzzles

Let’s say we pass in additional values, such as: 5 * 8, 5 * 9 and 5 * 10. However, within the call to format(), we are only referring to the values at index 0, index 1 and index 2. The values at indexes 3 and 4 are not used at all. What would happen when we run the code?

    >>> print("{0} * {1} = {2}".format(5,8,5*8,5*9,5*10))
    5 * 8 = 40

Would this throw an error? No, it does not. You can see that the additional values which are passed in, are conveniently ignored.

Let’s say instead of passing in a value of 2, we pass 4. What would happen?

    >>> print("{0} * {1} = {4}".format(5,8,5*8,5*9,5*10))
    5 * 8 = 50

5 * 10 is the value at index 4

Now let’s take a different scenario. We remove all the parameters passed to format(). However, inside the call to print(), we continue to say {0} * {1} = {4}. So we are trying to print the value at index 4, but are only passing two values to the function format(). What do you think will happen?

    >>> print("{0} * {1} = {4}".format(5,8))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    IndexError: tuple index out of range

It says IndexError, which means :“you are asking me to fetch the value at index 4, but only passing in two values. How can I do what you want?”

Let’s look at a few more things related to other data types. We try to format the following inside print(): {0} * {1} = {2}, and would pass in 2.5, 2, and 2.5 * 2 . Here, 2 is an integer value, but 2.5 is a floating point value. You can see that it prints 2.5 * 2 = 5.0. So this approach of formatting values with print(), works also with floating point data as well.

    >>> print("{0} * {1} = {2}".format(2.5,2,2.5*2))
    2.5 * 2 = 5.0

Now, are there are other types of data that format() works with? Yes, strings can join the party.

Let’s say over here, we do: print("My name is {0}".format("Ranga")). What would happen?

    >>> print("My name is {0}".format("Ranga"))
    My name is Ranga

Index 0 will be replaced with the first parameter to format().

Summary

In this step, we:

• Understood the behavior when the parameters passed to format():

  • Exceed the indexes accessed by print()

  • Are less than the indexes accessed by print()

  • Are of type integer, floating-point or string

Step 10: Introducing Variables

We are slowly making progress toward our main goal, which is to print the 5 multiplication table.

In the first statement, we are printing 5 * 1 = 5, and then changing the literals. To make it print 5 * 2 = 10, we are changing 1 to 2. Next, we are changing 2 to 3. How do we make it a little simpler, so that our effort is reduced?

    >>> print("{0} * {1} = {2}".format(5,1,5*1))
    5 * 1 = 5
    >>> print("{0} * {1} = {2}".format(5,2,5*2))
    5 * 2 = 10
    >>> print("{0} * {1} = {2}".format(5,3,5*3))
    5 * 3 = 15

Let’s try a different approach.

What would happen if you replace 1 with index, and 5 * 1 with 5 * index, and try to run it?

It gives an error! It says: “index is not defined”.

Let’s try and fix this, and execute index = 2. What would happen?

    >>> index = 2

Aha! This compiles.

    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 2 = 10

And this statement is printing 5 * 2 = 10.

Let’s try something else. Let’s make index = 3. What would happen?

    >>> index = 3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15

The same statement on being run, prints 5 * 3 = 15.

How can you check the value that index has? Just type in index.

    >>> index
    3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15

The index symbol we have used here, is what is called a variable.

In Python, it’s also called a name.

You can see that the value index referring to, can change over the duration of a program.

Initially, index was referring to a value of 1. later, index was referring to a value of 3.

Now, think about how you would print the entire table. All that you need to do, is start from 1, execute the same statement with print() and format(), to get output 5 * 1 = 5. Next, Change the value of index to 2, and then print the same statement. Next, index = 3, and print the same statement again.

    >>> index = 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 1 = 5
    >>> index = 2
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 2 = 10
    >>> index = 3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15

With the same statement print("{0} * {1} = {2}".format(5,index,5*index)), we are able to print different values. The value of index varies, but the code remains the same!

Variables make the program much more easier to read, as well as more generic.

Snippet-02: Classroom Exercise On Variables

Let’s do a simple exercise with variables.

We want to create three variables a, b and c. Let’s initially give them some values, say a value of 5 to a, 6 to b and 7 to c.

We want to get output of this kind: 5 + 6 + 7 = 18, without using the literal values.

You would want to use the values stored in the variables in a, b and c.

If you’re hard-coding, the way to do it is with print("5 + 6 + 7 = 18").

    >>> a = 5
    >>> b = 6
    >>> c = 7
    >>> print("5 + 6 + 7 = 18")
    5 + 6 + 7 = 18
    >>> print("5 + 6 + 7 = 18".format(a,b,c,a+b+c))
    5 + 6 + 7 = 18

The way you can do that is with code like this: print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c)).

    >>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
    5 + 6 + 7 = 18

How do you confirm we are accessing values stored in the variables?

Let’s change the values of a, b and c. Let’s make a = 6 , b = 7 , and c = 8 . Execute same statement.

    >>> a = 6
    >>> b = 7
    >>> c = 8
    >>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
    6 + 7 + 8 = 21

You can see the magic of variables at play here! Based on what values these variables are referring to, you can see that the output of the print statement changes.

Summary

In this step, we:

• Were introduced to variables, or names, in Python

• Observed how we could pass in values of variables to the format() function

Step 11: Puzzles On Variables

In the previous step, we were introduced to the concept of variables in Python.

We will start with looking at a few puzzles.

Snippet-01: Puzzles On Variables

What if I try to refer to a variable which is not yet created?

    >>> count
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'count' is not defined
    >>> print(count)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'count' is not defined

Before using a variable, you need to have it assigned a value. If you have not defined a variable before, then you cannot use it. Consider print(count), it does not know what count is. So it would throw an error, saying: “count is not defined, I have no idea what count is.”

Once you assign a value to a variable, you can use it.

    >>> count = 4
    >>> print(count)
    4

The statement count = 4 where we are creating a variable named count for the first time, is called a variable definition.

This is the first time you’re referring to a variable, and assigning a value to it.

Python will create a variable in its memory.

Variable names are case sensitive. count and Count are not the same thing.

    >>> Count
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Count' is not defined
    >>> count
    4

There are rules to follow while naming variables.

All variable names should either start with an alphabet , or an underscore _ . count, _count are valid. 1count is invalid.

    >>> 1count = 5
      File "<stdin>", line 1
        1count = 5
             ^
    SyntaxError: invalid syntax
    >>> count = 5
    >>> _count = 5
    >>> 1count
      File "<stdin>", line 1
        1count
             ^
    SyntaxError: invalid syntax
    >>> 2count
      File "<stdin>", line 1
        2count
             ^
    SyntaxError: invalid syntax

After the first symbol, you can also use a numeral in variable names.

    >>> c12345 = 5

To summarize the rules for naming variables.

• This should start with an alphabet (a capital or a small alphabet) or underscore.

• Starting the second character, it can be alphabet, or underscore, or a numeric value.

Summary

In this step, we:

• Understood that a variable needs to be defined before it is used

• Learned that there are certain rules to be followed while giving names to variables

Step 12: Introducing Assignment

In this step, we will look at an important concept in Python, called assignment. In previous steps, we created variables, like i = 5.

Snippet-01: Introducing Assignment

You can create other variables using whatever value i is referring to. If we say j = i, what would happen?

    >>> i = 5
    >>> j = i
    >>> j
    5

j would start referring to the same value that i is referring to. This statement is called an assignment.

Let’s try j = 2 * i.

    >>> j = 2 * i
    >>> j
    10

j refers to a value of 10

= has a different meaning in programming compared to mathematics.

In mathematics, When we execute j = i, it means j and i are equal.

In prgramming, the value of the expression on right hand side is assigned to the variable on the right hand side. Can you use a constant on the left hand side of an assignment? The answer is “No”!

    >>> 5 = j
      File "<stdin>", line 1
    SyntaxError: can't assign to literal

The Python Shell throws an error, saying “Can’t assign to literal”, as 5 is a literal.

Let’s create a couple of variables. num1 = 5 and num2 = 3. We would want to add these and create a fresh variable. Let’s say the name of the variable is sum.

    >>> num1 = 5
    >>> num2 = 3
    >>> sum = num1 + num2
    >>> sum
    8

Create 3 variables a, b and c with different values and calculate their sum.

    >>> a = 5
    >>> b = 6
    >>> c = 7
    >>> sum = a + b + c
    >>> sum
    18

We have just seen the mechanics of how assignment works in Python.

Summary

In this step, we:

• Learned what happens when you assign a value to a variable, which may or may not exist

• Discovered that literal constants cannot be placed on the left hand side of the assignment(=) operator

Step 13: Introducing Formatted Printing

Until now, we have been using the format() method to format and print values. Let’s see a better approach to printing values.

This is the approach we used until now.

    >>> a = 1
    >>> b = 2
    >>> c = 3
    >>> sum = a + b + c
    >>> print("{0} + {1} + {2} = {3}".format(a, b, c ,sum))
    1 + 2 + 3 = 6

Python has the concept of formatted strings. The syntax to use a formatted string is very simple - f"".

If we want to print the value of a variable a, we can use {a} in the text.

    >>> print(f"")
    >>> print(f"value of a is {a}")
    value of a is 1
    >>> print(f"value of b is {b}")
    value of b is 2

The variable within braces is replaced by its value.

You can use expressions in a formatted string. Example below uses {a+b}.

    >>> print(f"sum of a and b is {a + b}")
    sum of a and b is 3

This feature was introduced in a Python 3 release.

Let’s get back to the original problem we wanted to solve: printing 5 + 6 + 7 = 18, using formatted strings.

    >>> print(f"{a} + {b} + {c} = {sum}")
    1 + 2 + 3 = 6

You can see how easy it turns out to be!

Step 14: The PMT-Challenge Revisited

We want to print the 5-table from 5 * 1 = 5 onward, until we reach to 5 * 10 = 50. The best solution we have right now, is shown below:

Snippet-01:

    >>> index = 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 1 = 5
    >>> index = 2
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 2 = 10
    >>> index = 3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15
    >>> index = 4
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 4 = 20

Can we do something, to make sure that the code remains the same all the time, but the index value gets updated?

    >>> index = index + 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 5 = 25
    >>> index = index + 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 6 = 30
    >>> index = index + 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 7 = 35

We used index = index + 1 to increment index value.

If we execute these same two statements again and again, we can print the entire table! This is exactly what loops help us do: execute the same statements repeatedly.

The simplest loop available in Python is the for loop.

When we run a for loop, we need to specify the range of values - 1 to 10 or 1 to 20, and so on. range() function helps us to specify a range of values.

    >>> range(1,10)
    range(1, 10)

The syntax of the for loop is: for i in range(1, 10): .... Here, i is the name of the control variable. In Python, you need to put a colon, ‘:’, and in the next line give indentation.

    >>> for i in range(1,10):
    ...   print(i)
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9

You would see that it prints from 1 to 9.

When we run a loop in range(1, 10), 1 is inclusive and 10 is exclusive.The loop runs from 1 to the value before 10, which is 9.

The leading whitespace before print(i) is called indentation. We’ll talk about indentation later, when we talk about puzzles related to the for loop.

How can you extend this concept to solving our PMT-Challenge problem?

    >>> print(f"{5} * {index} = {5*index}")
    5 * 7 = 35

What we were doing earlier, was calling print() with a formatted string. Now we want to print this statement for different values of i.

How can you do that?

Let’s start with a simple example.

    >>> for i in range(1,11):
    ...   print(f"{i}")
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10

print(f"{i}") prints the value of i.

Now, how do we get it to print 5 * 1 = 5 to 5 * 10 = 50?

    >>> for i in range(1,11):
    ...   print(f"5 * {i} = {5 * i}")
    ...
    5 * 1 = 5
    5 * 2 = 10
    5 * 3 = 15
    5 * 4 = 20
    5 * 5 = 25
    5 * 6 = 30
    5 * 7 = 35
    5 * 8 = 40
    5 * 9 = 45
    5 * 10 = 50
    >>> 5 * 4 * 50
    1000

print(f"5 * {i} = {5 * i}") prints a specific multiple of 5.

Step 15: Loops

In a previous step, we took a major step in programming. We wrote our first for loop with Python. In this step, let’s try a few puzzles to understand the for loop even further.

The syntax of the for loop we looked at earlier was:

  for i in range(1, 10):
    print(i)

Snippet-01:

Let’s say we write a for loop, but don’t give a : after the range() method, to close the first line. What would happen?

    >>> for i in range(1,10)
      File "<stdin>", line 1
        for i in range(1,10)
                           ^
        SyntaxError: invalid syntax

Invalid syntax. A : is mandatory within the for loop syntax.

Let’s provide a : and in the next line, use print(i) without space before it (without indentation).

    >>> for i in range(1,10):
    ... print(i)
      File "<stdin>", line 2
        print(i)
            ^
    IndentationError: expected an indented block

Most other programming languages use open brace { and closed brace } as delimiters in a for loop. However, Python uses indentation to identify which code is part of a for loop, and which is not. So if we are writing the body of a for loop, we must use indentation, and leave atleast a single <SPACE>.

    >>> for i in range(1,10):
    ...   print(i)
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9

How do we execute two lines of code as part of the for loop?

    >>> for i in range(1,10):
    ...  print(i)
    ...  print(2*i)
    ...
    1
    2
    2
    4
    3
    6
    4
    8
    5
    10
    6
    12
    7
    14
    8
    16
    9
    18

We are indenting both statements with a space - print(i) and print(2*i).

When for loop has only one line of code, you can specify it right after the :

    >>> for i in range(2,5): print(i)
    ...
    2
    3
    4

However, this is not considered to be a good programming practice. Even though you may want to execute just one statement in a for loop, indentation on a new line is recommended.

Another best practice is to use four <SPACE>s for indentation, instead of just two. This would give clear indentation of the code.

    >>> for i in range(2,5):
    ...     print(i)
    ...
    2
    3
    4

Anybody who looks at the code immediately understands that this print() is part of the for loop.

Let’s say you only want to print the odd numbers till 10, which are 1, 3, 5, 7 and 9. The range() function offers an interesting option.

    >>> for i in range (1,11,2):
    ...   print(i)
    ...
    1
    3
    5
    7
    9

In for i in range(1, 11, 2), we pass in a third argument, called a step. After each iteration, the value of i is increment by step.

Summary

In this step, we:

• Looked at a few puzzles about the for loop, which lay emphasis on the following aspects of for:

  • The importance of syntax elements such as the colon

  • Indentation

  • Variations of the range() function

Step 16: Programming Exercise PE-BA-02

In the previous step, after initially exploring the Python for loop, we looked at a number of puzzles.

In this step, let’s look at a few exercises.

Exercises

1. Print the even numbers up to 10. We would want to print 2 4 6 8 10, using a for loop.

2. Print the first 10 numbers in reverse

3. Print the first 10 even numbers in reverse

4. Print the squares of the first 10 numbers

5. Print the squares of the first 10 numbers, in reverse

6. Print the squares of the even numbers

Solution 1

Instead of starting with 1, we need to start with 2. Each time, i it would be incremented by 2, and 2 4 6 8 and 10 would be printed.

    >>> for i in range (2,11,2):
    ...   print(i)
    ...
    2
    4
    6
    8
    10

Solution 2

We would want to print the numbers in reverse. Think about how you would do that using the range() function. We’d want go from 10, 9, 8, and so on up to 1.

    >>> for i in range (10,0,-1):
    ...   print(i)
    ...
    10
    9
    8
    7
    6
    5
    4
    3
    2
    1

The value to start with is 10. As we discussed earlier, the end value is exclusive. So to print from 10 to 1, we want to end one value which is 0. range(10, 0) seems to be what we need.

Usually these step value is positive, but we need to go backwards from 10. Hence, we would give a step value of -1.

Solution 3

Now, let’s print the first 10 even numbers in reverse.

    >>> for i in range (20,0,-2):
    ...   print(i)
    ...
    20
    18
    16
    14
    12
    10
    8
    6
    4
    2

Solution 4

Next, we would want to print the squares of the first 10 numbers.

    >>> for i in range (1,11):
    ...     print(i * i)
    ...
    1
    4
    9
    16
    25
    36
    49
    64
    81
    100

Solution 5

Let’s print the squares in the reverse order.

    >>> for i in range (10,0,-1):
    ...     print(i*i)
    ...
    100
    81
    64
    49
    36
    25
    16
    9
    4
    1

Solution 6

Print the squares of the even numbers. How to do that?

    >>> for i in range (10,0,-2):
    ...     print(i*i)
    ...
    100
    64
    36
    16
    4

The key part is using a step of -2

We leave it as an exercise for you, to print squares of odd numbers.

Summary

In this video, we: * Tried out a few exercises involving the for loop, by playing around with printing sequences of numbers.

• Used the for loop to simplify the solution to the PMT-Challenge problem.

Step 17: Review: The Basics Of Python

It must have been a roller-coaster ride to solve the multiplication table challenge so far. If you’re new to programming, there are a wide range of topics and concepts, that you would have learned during this small journey.

Let’s quickly revise the important concepts we have learned during this small journey.

• 1, 11, 5, … are all called literals because these are constant values. Their values don’t really change. _Consider 5 _ 4 _ 50`. This is an expression. `_`is an operator, and`5`, `4`and`50 are operands.

• The name i in i = 1, is called a variable. It can refer to different values, at different points in time.

• range() and print() are in-built Python functions.

• Every complete line of code is called statement. The specific statement print(), is invoking a method. The other statement which we looked at earlier, was an assignment statement. index = index + 1 would evaluate index + 1, and have the index variable refer to that value.

• The syntax of the for loop was very simple. for var in range(1, 10) : ..., followed by statements you would want to execute in a loop, with indentation. For the sake of indentation we left four <SPACE>s in front of each statement inside the for loop.

So that, in a nutshell, is what we have learned over the course of our first section.

Chapter 03 - Introducing Methods

In the last section, we introduced you to the basics of python. We learned those concepts by applying them to solve the PMT-Challenge problem. The code below is what we ended up with as we solved that chellenge.

Snippet-01: Current Solution To PMT-Challenge

    >>> for i in range (1,11):
    ...   print(f"8 * {i} = {8 * i}")

If we wanted to change the code to print the 7 table, we need to change the value 7 used in the for loop, to 8. It’s simple, but still not as friendly as you would like.

    >>> for i in range (1,11):
    ...   print(f"7 * {i} = {7 * i}")

To print a 7 table, it would be awesome if could say print_multiplication_table, and give a value of 7 beside it, and it would do the rest:

    >>> print_multiplication_table(7)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'print_multiplication_table' is not defined
    >>> print_multiplication_table(8)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'print_multiplication_table' is not defined

Similarly, print_multiplication_table(8), could print the multiplication table for 8!

To be able to do this, we need to create a method, or a function. Creating a method makes the code reusable, and we can invoke that method very easily by passing arguments.

In this section, we take an in-depth look at methods.

Step 01: Defining Your First Method

Methods are very important building blocks in Python programming. In this step, we will create a simple method that prints "Hello World", twice.

Snippet-01:

When we talk about a method, we need to give it a name. We are already using an in-built Python method here, which is print().

    >>> print("Hello World")
    Hello World
    >>> print("Hello World")
    Hello World

Similar to that, we need to give a name to our body of code. Let’s say the name is print_hello_world_twice.

The syntax to create a method in Python is straightforward:

• At the start, use the keyword def followed by a space.

• Followed by name of the method - print_hello_world_twice.

• Add a pair of parenthesis: ().

• This is followed by a colon : (similar to what we used in a for loop).

  >>> def print_hello_world_twice():
  ...     print("Hello World")
  ...     print("Hello World")
  ...